1. Field of the Invention
The present invention relates to a solid-state imaging device and, more particularly, to a solid-state imaging device capable of compensating the interface state of the surface of a semiconductor substrate with photoelectric converting sensors formed thereon.
2. Description of the Related Art
Video cameras and digital still cameras are commonly equipped with a solid-state imaging device composed of CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) image sensors. Solid-state imaging devices of any kind have important problems with sensitivity improvement and noise reduction.
Noise involved in solid-state imaging devices is due to dark current. Dark current is a very small current that occurs in the absence of incident light. In other words, the interface of the substrate of the light receiving surface has minute defects that generate charges (or electrons) behaving like signals even though there exist no pure signal charges originating from photoelectric conversion of incident light. Dark current also originates from the interface state at the interface between the sensor and the film of upper layer.
One of the known ways of suppressing dark current originating from the interface state is by employing the so-called HAD (Hole Accumulated Diode) structure. HAD is a photodiode of buried type which has a hole accumulation layer or a P+ layer on the sensor (e.g., photodiode).
As shown in FIG. 4, which is a sectional view, the HAD structure is composed of an N-type semiconductor substrate 101 and a buried photodiode D formed thereon. The photodiode D is composed of a P-well diffusion layer 102 and an N-type diffusion layer 103, which are sequentially arranged upward from the surface of the substrate 101. The top of the photodiode D is coated with a hole accumulation layer 104 which is a P+ impurity diffusion layer. The hole accumulation layer 104 is further coated with an insulating film 105, whose surface functions as a photodetecting surface.
The above-mentioned HAD structure offers the following advantage. Even though charges (or electrons) due to the interface state occur at the interface (or the surface of the hole accumulation layer 104) between the insulating film 105 and the semiconductor substrate 101, the hole accumulation layer 104 formed on the buried photodiode D prevents such electrons from flowing from the N-type diffusion layer 103 (as a constituent of the photodiode D) into the charge accumulation part but allows them to flow into the hole accumulation layer 104 (or P+ layer) abounding with holes in which they disappear. Thus, the negative charges arising from the interface state do not generate dark current to be detected.
The HAD structure mentioned above can be applied to either CCD image sensors or CMOS image sensors. It can also be applied to related-art image sensors of top-side illumination type as well as new image sensors of back-side illumination type (disclosed in Japanese Patent Laid-open No. 2003-338615).
The HAD structure is usually produced in the following manner. The semiconductor substrate 101 is provided with the photodiode D thereon. The photodiode D is coated with the insulating film 105 which is a thermally oxidized silicon oxide film or CVD (Chemical Vapor Deposition)-oxidized silicon oxide film. The insulating film 105 undergoes ion implantation so that P-type impurities, such as boron (B) and boron difluoride (BF2), are introduced into the surface layer of the semiconductor substrate 101 (or the surface layer of the photodiode D). The thus introduced impurities are activated by annealing so as to form the hole accumulation layer 104 (which is a P+ impurity diffusion layer) near the interface.